The invention relates to a method for structuring a photoresist layer.
In semiconductor technology, photolithographic methods for producing integrated circuits on a semiconductor substrate play a central role. In these methods, photoresist layers are typically applied onto the surface of a substrate that is to be structured, and are subsequently structurally exposed with radiation from a suitable wavelength range. Here, the structural exposure takes place using a lithography mask, through which the structure that is to be transferred into the substrate is predetermined. The exposed regions of the photoresist layer are chemically modified by the exposure, thus modifying their polarity. In this way, the exposed and unexposed regions of the photoresist have different solubilities in relation to a corresponding developer. This fact is exploited in the subsequent developing step for the selective removal of the exposed or unexposed regions. The regions of the photoresist layer remaining on the substrate are used in the following structuring step as a mask, which protects the substrate layer located underneath it from a wearing away of material or modification of the material. Such a structuring step can be for example a plasma etching, a wet-chemical etching, or an ion implantation.
Chemically strengthened resists (chemical amplification resists; CAR) have proven particularly effective as photoresists both for one-layer resists, which can be wet-developed, and for two-layer resist systems, which can be partly or entirely dry-developed. In these photoresists, photoacid generators are used as photosensitive compounds. An overview of the subject is given by H. Ito in Solid State Technology, July 1996, p. 164 ff. In a selected group of these systems, the solubility modification is achieved using the principle of acid-catalyzed separation or decomposition. This principle can be used both in resists that operate positively and those that operate negatively.
In the case of a positive resist, a polar molecular group, for example a carboxylic acid, is formed during a heating step from an unpolar chemical group, for example a carboxylic acid tert.-butyl ester group, in the presence of a photolytically produced acid. Further examples of unpolar xe2x80x9cblockedxe2x80x9d groups that can be converted into corresponding polar groups through acid-catalyzed reactions include the tert.-butoxycarbonyloxy (t-BOC) or acetal groups. Through the conversion of the unpolar groups into the corresponding polar groups, the resist undergoes a change of polarity in the previously irradiated regions, and thus becomes soluble in relation to a polar, aqueous-alkaline or diluted alkaline developer. In this way, the exposed regions of the photoresist can be removed selectively by the developer.
In contrast to this, in negative resists the exposure effects a reduction of the solubility of the photoresist in the exposed regions. In order to achieve this, as a rule photoresists that operate negatively have groups that can be cross-linked and which can form cross-linkage reactions under the influence of radiation. Through the cross-linkage, the solubility of the exposed regions of the photoresist in a corresponding developer is reduced. The cross-linkable groups can either be bound directly to the base polymer or can be present in the photoresist as separate cross-linkage components. In chemically reinforced, negatively operating photoresists, groups that can be acid-catalytically cross-linked are used that are activated by the photolytically released acids.
Due to the ever-increasing scale of integration in semiconductor technology, the precision with which the resist profile can be produced on a surface to be structured after development is of a decisive importance. On the one hand, the resist profile is physically predefined in unambiguous fashion by the light distribution during the exposure. On the other hand, it is chemically transferred into the resist layer through the distribution of the components that are photochemically produced during the exposure.
However, due to the physical-chemical properties of the resist materials, a completely unfalsified transfer of the pattern predetermined by the lithography mask into the resist profile is not possible. In particular, interference effects and light scattering in the photoresist play an important role here. But the steps following the exposure, such as for example the developing, also influence the quality of the resist profiles. The quality of the resist profiles is essentially determined by the profile edges. In order to achieve, in subsequent process steps, a surface structuring that is as precise as possible, for example, to ensure a maximally high resolution, it would be ideal if nearly vertical, smooth profile edges could be obtained in the resist profile after the developing of the photoresist.
The light intensity profile that arises in the photoresist during the exposure has a negative effect on the steepness of the profile edges that is to be achieved. This characteristic intensity profile, also called the xe2x80x9ca real imagexe2x80x9d, results from the light scattering and light absorption that occur in the photoresist during the exposure. Because the photoresist absorbs a certain portion of the incident radiation, the observed radiation intensity decreases as the layer thickness in the photoresist increases. As a consequence, the regions close to the surface of the photoresist layer are more strongly exposed. In negative resists, this results in a stronger cross-linkage in the regions close to the surface, which thus have a lower solubility than the regions further from the surface. In the subsequent developing of the exposed photoresist, the regions of the photoresist layer far from the surface are therefore more strongly dissolved than are the regions close to the surface and situated thereabove; and this will cause the quality of the profile edges, and thus the resolution, to be worsened.
A further problem in photoresists in which a solubility modification is achieved through cross-linkage reactions is that a three-dimensional network polymer is formed in the photoresist layer in the course of the cross-linkage reaction. In comparison to the original, linear layer-forming polymer, this network polymer has a modified development characteristic, which makes the uniform resolution of the photoresist layer along the exposure boundary more difficult. This leads to xe2x80x9crough,xe2x80x9d for example imprecisely defined, for example lacerated, profile edges.
The present invention is therefore based on the object of providing a method for structuring a photoresist layer that reduces, or avoids entirely, the disadvantages described above. In particular, it is the object of the present invention to provide a method with which a high degree of precision is achieved in transferring the structure predetermined by the lithography mask into a negatively operating photoresist layer.
This object is achieved by the method for structuring a photoresist layer comprising the steps of: preparing a substrate having a photoresist layer which applied at least in subregions, the photoresist layer comprising a film-forming polymer that comprises molecular groups that can be converted into alkali-soluble groups through acid-catalyzed separation reactions, and a photobase generator that releases a base when irradiated with light from a defined wavelength range; irradiating the photoresist layer in the subregions with light from the defined wavelength range; contacting the photoresist layer with an acid over a defined period of time, during which the acid diffuses into the photoresist layer; heating the photoresist layer to a temperature at which the acid-catalyzed separation reaction takes place; and developing the photoresist layer. Further advantageous specific embodiments, constructions, and aspects of the present invention will be apparent from the claims and from the following description.